1. Field of the Invention
This invention relates to the field of turbo-charger surge detection.
2. Description of the Related Art
An internal combustion engine may include a super-charger or a turbo-charger for compressing intake air prior to delivery to the combustion chambers. A super-charger is typically belt or gear-driven, while a turbo-charger has a turbine which is driven by the engine's exhaust gases. The belt or gears, in the case of a supercharger, or the turbine, in the case of a turbocharger, drives a compressor which compresses the intake air. The compressor, which may be a centrifugal or rotary pump, receives air to be compressed on an inlet side and supplies the air to the combustion chambers from an outlet side. The difference in pressure of the outlet side relative to the inlet side is termed the pressure ratio, and represents the amount of boost the compressor is supplying to the intake air.
In FIG. 1 is shown a compressor map 100 for a turbocharger compressor. Pressure ratios 102 are plotted on the vertical axis and rates of flow 104 are plotted on the horizontal axis. As may be seen in FIG. 1, an operating region 106 of the compressor is bounded on the left side of the compressor map by a surge line 108.
Surge occurs when rate of flow 104 through the compressor is too small to support the prevailing pressure ratio 102. Surge line 108 represents this condition for various rates of flow 104. When rate of flow 104 is too small to support the prevailing pressure ratio 102, the air flow will cavitate, separating from the suction side of the blades or vanes of the compressor wheel and reversing air flow through the compressor until pressure ratio 102 is reduced. If the surge conditions continue to prevail, pressure ratio 102 will build up again and the cycle will be repeated. This cycle of rising and falling pressure ratios 102 may continue at a substantially fixed frequency. Surge makes a popping noise and stresses the piping between the turbo-charger and the inlet to the engine. The popping noise is called surging or barking. Customers using a truck that has this surging or barking dislike it and are afraid that it is causing damage to their truck and the engine.
The interaction between the turbo-charger, such as a variable geometry turbo-charger, and an EGR system may exacerbate a back flow of gas through the turbo-charger compressor. An EGR system may provide exhaust gas downstream of the compressor to avoid soaking the compressor in corrosive exhaust gases. The recirculated exhaust gas entering the air flow downstream of the compressor may add to the pressure at the outlet of the compressor, raising pressure ratio 102 artificially and promoting surge.